1. Field of the Invention
The present invention relates generally to gas turbine blades and, more particularly, to hollow gas turbine blades formed by a casting operation that leaves core printout holes therein. Specifically, the invention relates to a method for closing core printout holes in superalloy gas turbine blades.
2. Description of the Related Art
Turbine blades are employed in different regions of combustion turbine engines. As is known in the relevant art, such combustion turbine engines typically include a compressor stage, a combustor stage, and a turbine stage. Air is drawn into the engine and compressed by the compressor stage, with fuel being mixed with the compressed air and the fuel/air mixture being combusted in the combustor stage. The hot combusted gases then flow past the turbine stage and thereafter exit the engine.
The compressor and turbine stages of the engine typically include a plurality of turbine blades that are mounted on a common rotating shaft. The compressor and turbine stages each additionally include one or more stationary vanes or stators that include non-moving turbine blades that cooperate with the turbine blades mounted on the rotating shaft to compress air and to derive mechanical power from high velocity gases.
Since the turbine blades, both moveable and stationary, operate in a high temperature environment, such blades are typically formed in a casting operation to include a hollow cavity. The cavity receives cooling air during operation of the combustion engine to provide a cooling effect to the blades and to control the operating temperature thereof. The hollow cavity is cast into the blade by providing a core within the blade mold. The core is retained within the mold by one or more ceramic rods that extend from the core to the inner surface of the mold itself for retaining the core in a given position within the mold. A molten alloy is then poured into the mold with the core disposed therein, whereby the core prevents the flow of the molten alloy within certain regions of the mold and ultimately results in a hollow region within the finished turbine blade that can receive the beneficially cooling air therein. During the casting operation, however, the ceramic rods that retain the core in position within the mold likewise prevent the flow of the molten alloy such that the finished turbine blade additionally includes one or more core printout holes resulting from the legs. Such core printout holes must be sealed prior to use of the turbine blade, otherwise the cooling air introduced into the hollow core will undesirably flow out of the printout hole without providing the needed beneficial cooling effect to the turbine blade.
Previous methods and apparatuses employed to seal such core printout holes have met with only limited success due to the difficulty of forming a seal having sufficient internal strength and being bonded strongly enough to the turbine blade to withstand the typical operating environment. As is known in the relevant art, such turbine blades typically are manufactured out of a xe2x80x9csuperalloyxe2x80x9d that typically is of a nickel base that is alloyed with other materials such as aluminum, titanium, and chromium in various combinations and proportions, although numerous other alloys can be used for the manufacture of turbine blades. Such superalloys typically include nickel aluminide intermetallic crystals known as a xe2x80x9cgamma primexe2x80x9d that are extremely brittle and are precipitated within a solid solution that makes up the turbine blade. Welding of such nickel-based superalloy materials is extremely difficult and often results in cracking and microfissuring due to strain age and liquation cracking. Any such welding is almost exclusively done with low strength weld filler materials since the higher strength fillers are more prone to cracking. While relatively low strength filler materials can, in certain circumstances, be employed to fill core printout holes without welding, such materials typically do not possess the strength of the base metal of the turbine blade, which thus often limit the utility of turbine blades having core printout holes that are repaired in such fashion. While Liquid Phase Diffusion Sintering (LPDS) methods can be employed in sealing core printout holes, such methods are of limited strength and are often expensive and require reworking and non-destructive examination to determine the adequacy thereof. A need thus exists for a method of sealing core printout holes whereby the seal is of sufficient internal strength and is attached to the blade securely enough to withstand the punishing environment typically found within the interior of a combustion turbine engine.
In accordance with the foregoing, an aspect of the present invention is to provide a method of closing a core printout hole in a superalloy turbine blade, the general nature of which can be stated as including the steps of providing a superalloy plug having a threaded peripheral configuration, machining the hole to have a threaded configuration that is structured to receive the plug therein, threading the plug in the hole, and bonding the plug to the turbine blade.
Another aspect of the invention is to provide a method of closing a core printout in a superalloy turbine blade, the general nature of which can be stated as including the steps of providing a superalloy plug having a peripheral configuration forming the hole to have a configuration that is structured to receive the plug therein, receiving the plug in the hole applying a bonding catalyst to at least one of the plug and the turbine blade, and forming a joint between the plug and the turbine blade.
A further aspect of the present invention is to provide a method of closing a core printout hole in a superalloy turbine blade having a first coefficient of thermal expansion, the general nature of which can be stated as including the steps of providing a superalloy plug having a peripheral configuration and having a second coefficient of thermal expansion that is greater than the first coefficient of thermal expansion, forming the hole to have a configuration that is structured to receive the plug therein, receiving the plug in the hole, and bonding the plug to the turbine blade.